In general, an oxidative hair color is composed of a first pack, which contains an oxidative dye precursor (color-developing substance) as a dye ingredient, a coupling substance and the like, and a second pack containing an oxidizing agent such as hydrogen peroxide, and these first and second packs are mixed together immediately before applying the oxidative hair color to the hair. As an alkaline condition is preferred in an oxidation step of the dye precursor from the viewpoint of color developing property, the first pack is generally formulated to be alkaline such that alkalinity is also maintained after the mixing. In a hair dyeing treatment, the hair is, therefore, exposed to an alkali and the oxidizing agent such as hydrogen peroxide so that the hair is unavoidably damaged.
With a view to overcoming the above-described problem, proposals have been made including: hair coloring techniques each of which makes use of a catalyst to allow a sufficient dyeing reaction to proceed even with a small amount of an oxidizing agent or with a weak oxidizing agent (U.S. Pat. No. 5,368,610); and hair coloring techniques each of which permits induction of an oxidation reaction with oxygen in the air without using any oxidizing agent. These air-oxidative hair coloring techniques include inter alia one making use of an oxidase such as laccase (WO 99/36034), one employing 2,4-diaminophenol which is an oxidative dye of low oxidation potential (JP-A-07082120 or JP-A-02053715), and one making use of an indoline which is a melanine precursor (JP-A-05507106).
The technique disclosed in U.S. Pat. No. 5,368,610, however, uses an oxidizing agent as a matter of fact and hence, cannot fully avoid a damage to hair. Concerning its hair dye composition, there is no choice other than formulating the composition into a two-pack, mixing at-need form. Further, the dyeing procedure is irksome. The process making use of an oxidase, on the other hand, is accompanied by a problem that, as the enzyme has a high molecular weight, its dye-oxidizing action can be hardly allowed to extend into hair fibers and no sufficient dyeing power is available and also by a problem that limitations are imposed on a surfactant, a solvent and the like, which are to be used in combination upon formulation, to avoid losing the activity of the enzyme. When 2,4-diaminophenol, a dye of low oxidation potential, is used, there is a concern about the safety of this compound. Use of an indoline, on the other hand, involves a limitation that color variations are limited because usable dyes are limited by the indoline itself.